Probes launch to study moon's interior

Update on 29 December&colon; NASA’s twin GRAIL spacecraft are now almost ready to begin scanning the moon. GRAIL-A is scheduled to be placed in orbit beginning at 1621 Eastern Standard Time on 31 December, and GRAIL-B at 1705 EST on 1 January.

Original article, posted 10 September 2011

The man in the moon is about to have his innards’ scanned. NASA has launched two probes that will study the moon’s internal structure in unprecedented detail, shedding light on whether a second moon crashed into it long ago.

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The probes, together called GRAIL (Gravity Recovery and Interior Laboratory), lifted off on Saturday at 0908 EDT from Cape Canaveral Air Force Station in Florida. Lift-off was originally set for 8 September but was postponed due to high winds.

After they reach the moon in about four months’ time, the probes will measure slight variations in the strength of the moon’s gravity, which isn’t uniform due to the uneven distribution of matter inside it.

The moon’s lumpy gravity field will cause slight differences in the force of gravity on the two probes, which will orbit the moon in single file, causing them to move closer or farther from each other. Monitoring the distance between them will produce lunar gravity maps with at least 100 times the resolution of previous measurements, including maps made by Japan’s Kaguya mission.

Impact scars

Lumpiness revealed by the maps should reveal more about the moon’s history, since it is thought to represent scarring from past impacts. For example, it was recently proposed that Earth once had a second moon that crashed onto our moon’s far side, explaining why the crust is much thicker there.

Better mapping of the crust by GRAIL could help determine if that theory is valid, says the mission’s chief scientist, Maria Zuber of MIT.

The maps might contain surprises that “turn our understanding of how the moon and other terrestrial planets formed on its ear”, she says.

Painstaking corrections

The maps could also ease navigation for any robotic or human missions that might be sent to the moon in the future. If not properly accounted for, the moon’s lumpy gravity field can cause spacecraft to drift off course.

GRAIL’s gravity maps will not only be the best ever made of the moon – they will be the best made of any solar system body. That’s because the GOCE satellite orbiting Earth, which has equally sensitive instruments, orbits Earth about 10 times higher than GRAIL orbits the moon, to avoid drag from our planet’s atmosphere. That means GRAIL can get more detailed measurements of the moon than GOCE can of Earth.

But attaining GRAIL’s intended precision will require painstaking calculations to correct for tiny sources of error, including the pressure of sunlight on the probes’ solar panels and the drifting of tectonic plates on Earth, which will slightly shift the position of radio dishes on the ground used to track the spacecraft.